This invention relates in general to athletic running and playing surfaces. More specifically, it relates to a surface composed of an array of modules having a laminated construction. The present invention is an improvement over the invention described in applicants' aforementioned applications.
Known athletic playing surfaces, particularly outdoor surfaces such as running tracks, football fields, and basketball courts are typically dirt, grass, asphalt or gravel over dirt. More recently, such surfaces have been constructed with a layer of a resilient synthetic plastic material, usually polyurethane, laid over a rigid substrate such as the asphalt or concrete. These plastic materials, such as the product sold under the trade designations "Astroturf", "Chem-Turf" or "Tartan" generally have a uniform composition and thickness when used as an athletic playing surface. All of these surfaces are characterized by a relatively high degree of rigidity (a low degree of vertical compliance). This characteristic results in a high incidence of injuries such as shin splints and foot injuries due to the high collision forces generated by the human leg striking a rigid surface when running. These problems are, of course, accentuated for competitive runners and those who are not in their best physical condition.
While these plastic surfaces enjoy a high degree of commercial success, they are not entirely satisfactory. First, while relatively thick resilient layers can reduce running induced injuries, the resilient material is usually only thick enough to enhance traction, reduce damage to the underlying surface from shoe spikes and reduce maintenance. In such thin layers, typically 3/8 to 1/2 inch, the running surface has been found to produce a relatively high level of injuries. Second, when the plastic layers are thick enough to significantly reduce injuries, they are poor running surfaces due to a relatively large degree of lateral compliance or low resistance to horizontal shear under an applied horizontal force and a high dependance of vertical compliance on the foot contact area, e.g., full foot versus only toe or heel contact. Third, they are comparatively expensive to install, particularly in thick layers necessary to achieve a relatively large degree of compliance. Fourth, when these plastic materials are used outdoors sunlight, temperature cycling, and exposure to adverse climatic conditions cause them to harden.
One solution to some of the foregoing problems that is commonly utilized in outdoor football fields is to place a resilient pad between the upper synthetic plastic layer and the underlying rigid substrate. While the resulting surface does have an enhanced softness, the degree of vertical compliance is not large enough to significantly reduce injuries or to provide what the applicants have found to be an optimal degree of compliance. Another problem with this pad design is that the resultant structure has a rather large degree of horizontal compliance (susceptibility to lateral shear). This is highly undesirable for running because (1) the energy transiently stored in the lateral deformation of the material is not returned to the runner and (2) foot control is poor. In addition, the pad material (often foam rubber), when compressed, is characterized by a generally low level of efficiency in transiently storing and returning energy to the runner. Further, in outdoor applications where the uppermost layer is a porous, woven synthetic material that simulates grass, rain can completely saturate the foam pad. It usually requires 24 to 48 hours of dry weather to restore the surface to its normal condition.
Heretofore conventional indoor athletic surfaces such as running tracks, basketball courts, and enclosed racket courts have used an extremely stiff upper surface laid on elongated support members or "sleepers". Many older tracks and basketball courts use stiff planks of hardwood that are interlocked with one another and secured to the sleepers. More recent running track designs have used other surface materials such as plywood panels over-laid with resilient materials of the type discussed above. In either case, it was assumed heretofore that the surface should be rigid (have an extremely low vertical compliance) to yield the best performance. Some other track designs have used plywood panels supported on several 2.times.4 inch wooden beams oriented perpendicular to the running direction. Such tracks provide some vertical compliance, but the degree of compliance varies greatly depending on whether or not a runner lands over a support beam. In these prior art surfaces, it is common for the compliance, as measured by a load deflection test, to vary over the surface by a factor of 10 (a non-uniformity of 1,000%) or more. This makes it difficult for a runner to maintain a uniform stride.
U.S. Pat. Nos. 1,693,655; 3,114,940; 3,045,294; and 3,271,916 and U.K. Pat. Nos. 1,113,244 and 1,478,850 describe floor constructions where hardwood boards or panels are supported on sleepers, but which also have a yielding material to provide some degree of cushioning. While these arrangements do provide some "give" to the floor, they have disadvantages when used as an athletic surface, particularly a running track. First, the yielding materials described in these patents are not highly resilient and therefore they absorb a substantial portion of the athlete's vertical kinetic energy each time he impacts on the surface. Second, for most of these constructions the compliance of the surface is not uniform. Third, there is no appreciation in this prior art of a general interrelationship between the vertical compliance of the running surface and the running speeds attainable on that surface other than the long accepted understanding that the hardest surface produces the fastest speed.
Further, the advantages of the yielding material in these constructions are overshadowed by other aspects of the construction, particularly the construction of the upper, wooden layer. In the '655, '940, and '294 patents the upper surface is formed by conventional interfitted (i.e. tongue-in-groove) floor boards or boards that are interlocked through flanged support rails. In the '916, '244, and '850 patents the upper surface is formed by larger panels which are mechanically coupled to one another. Because all of these upper layers are thus interconnected, they each (1) present a large apparent mass to the runner and (2) feed energy across the floor boards resulting in a phenomenon known as "cross-talk". As an example of cross-talk, if a runner lands on one end of a board or panel that is supported on a sleeper near its mid point, the board or panel can act like a lever causing the opposite end of the board or panel to accelerate upwardly or to have an increased resistance to the impacting foot of another runner. In addition, the impact of a runner on a continuous or interlocked surface can generate bending waves in the surface which propagate energy away from the runner and make the surface noisy. These phenomena contribute to the large variations in the response of conventional surfaces.
All of the foregoing designs for athletic surfaces also suffer from a number of other disadvantages. First, even those designs which offer some degree of vertical compliance in the surface provide no way of adjusting the degree of that compliance. Second, many of the surfaces mentioned above, particularly those involving wooden panels or flooring, are not suitable for outdoor use. Third, none of the designs described above are suitable for retro-fitting an existing playing surface such as a running track or a basketball court to provide an optimal degree of vertical compliance. Fourth many of the constructions utilizing a yielding material involve special hardware, skilled construction techniques, and are generally expensive to manufacture. Finally, none of the designs described above allow the upper surface of the floor to be replaced readily to accommodate different uses of the playing surface.
It is therefore a principal object of this invention to provide an athletic playing surface construction that provides an optimal degree of vertical compliance in the surface which enhances the running speed of athletes performing on the surface, reduces the likelihood of injuries, and is comfortable to run upon.
Another object is to provide a playing surface with the foregoing advantages that has a vertical compliance which is highly uniform over its surface and is independent of the foot contact area.
Another object of the invention is to provide a surface which provides an extremely high resistance to lateral shear.
A still further object is to provide an athletic playing surface with the foregoing advantages that provides a low effective vertical mass and substantially eliminates cross-talk.
Yet another object of the invention is to provide an athletic playing surface with a vertical compliance that is conveniently adjustable.
Another object is to provide an athletic playing surface with the foregoing advantages which allows the upper layer or layers of the surface to be replaced to accommodate the different uses of the surface.
A still further object of the invention is to provide an athletic playing surface construction which can be used either indoors or outdoors.
Yet another object of the invention is to provide an athletic playing surface construction which can "retro-fit" many existing, conventional surfaces.
A still further object of the invention is to provide a playing surface construction which lends itself to prefabrication resulting in a reduction in on-site construction costs.